Two-Wire Process Control Loop Diagnostics

1. A diagnostic device for coupling to a two-wire process control loop of an industrial process control or monitoring system, comprises: digital communication circuitry configured to receive a digital communication signal from the two-wire process control loop, the digital communication signal comprising a digitally modulated analog signal on the two-wire process control loop which is modulated to a plurality of discrete analog signal levels representative of digital values; and diagnostic circuitry configured to diagnose operation of the two-wire process control loop based upon the digitally modulated analog signal.

2. The apparatus of claim 1 wherein the digital communication circuitry includes a sense resistor.

3. The apparatus of claim 1 including an analog to digital convertor configured to digitize the digitally modulated analog signal.

4. The apparatus of claim 1 wherein the diagnostic circuitry monitors amplitude of the digitally modulated analog signal.

5. The apparatus of claim 1 wherein the diagnostic circuitry monitors wave shape of the digitally modulated analog signal.

6. The apparatus of claim 1 wherein the diagnostic circuitry monitors a bit error rate (BERT) of digital transmissions on the two-wire process control loop.

7. The apparatus of claim 1 wherein the diagnostic circuitry monitors impedance of the two-wire process control loop.

8. The apparatus of claim 7 wherein the impedance is monitored by receipt of a high frequency signal on the two-wire process control loop.

9. The apparatus of claim 1 wherein the diagnostic circuitry compares a parameter of the digitally modulated analog signal to a stored value and responsively provides a diagnostic output.

10. The apparatus of claim 1 wherein the diagnostic circuitry correlates diagnostic information based upon the digitally modulated analog signal and a particular device on the two-wire process control loop which transmitted the digitally modulated analog signal.

11. The apparatus of claim 1 wherein the diagnostic circuitry performs diagnostics on a device coupled to the two-wire process control loop.

12. The apparatus of claim 1 wherein the diagnostic circuitry performs diagnostics on wiring of the two-wire process control loop.

13. The apparatus of claim 1 including a display configured to display diagnostic information.

14. The apparatus of claim 1 including a process interface for sensing or controlling a process variable of the process.

15. The apparatus of claim 1 wherein the diagnostic device is configured to mount in the field of the industry process control or monitoring system.

16. The apparatus of claim 1 wherein the digital communication circuitry and the diagnostic circuitry are powered with power received from the two-wire process control loop.

17. The apparatus of claim 1 wherein the diagnostic circuitry diagnoses operation of a process device of the two-wire process control loop.

18. A method for diagnosing a two-wire process control loop of the type used in an industrial process control or monitoring system, comprising: receiving digital communication signals from a plurality of devices coupled to the two-wire process control loop, the digital communication signals comprising a digitally modulated analog signal which is modulated to a plurality of discreet analog signal levels representative of digital values; measuring a property of the digitally modulated analog signal; diagnosing operation of the two-wire process control loop based upon the measured property of the digitally modulated analog signal; and providing an output based upon the diagnosing.

19. The method of claim 18 wherein measuring a property comprises monitoring amplitude of the digitally modulated analog signal.

20. The method of claim 18 wherein measuring a property comprises monitoring wave shape of the digitally modulated analog signal.

21. The method of claim 18 wherein measuring a property comprises monitoring a bit error rate (BERT) of digital transmissions on the two-wire process control loop.

22. The method of claim 18 wherein measuring a property comprises monitoring impedance of the two-wire process control loop.

23. The method of claim 22 wherein measuring a property comprises monitoring by receiving a high frequency signal on the two-wire process control loop.

24. The method of claim 18 including comparing a parameter of the digitally modulated analog signal to a stored value and responsively providing a diagnostic output.

25. The method of claim 18 includes correlating diagnostic information based upon the digitally modulated analog signal and a particular device on the two-wire process control loop which transmitted the signal.